The present invention is directed to sealed zinc/halogen compound electrochemical cells, and more particularly to these cells containing auxiliary electrodes which function to retard pressure build-up. The present invention is also directed to the method of retarding pressure build-up in these cells by the use of auxiliary electrodes.
Sealed electrochemical cells are of considerable importance as an efficient and durable source of energy. Due in part to their self-containment, they often have long shelf life, high energy efficiency and reasonable charge and discharge rates. Since the cells are sealed, little or no maintenance is required and this is perhaps critical in many applications. Among the many types of sealed cells which have various combinations of the foregoing advantages are the alkaline batteries, e.g., nickel-cadmium cells and lead-acid batteries. Heretofore, the zinc-halogen compound batteries, e.g., the zinc-Cl.sub.2 and zinc-Br.sub.2 batteries, which were used in a sealed configuration required venting and maintenance, and could not, therefore, be used in a totally sealed, maintenance-free configuration.
While many types of sealed cells have significant commercial possibilities, they often incurred a common problem. Specifically, one type of gas or another was generated and built up inside the cell during operation. This gas buildup could have resulted in rupture of the seal and/or diminished capabilities of the cell in terms of output. The problem has been dealt with in many ways including the use of venting devices, as well as the use of electrical devices and the use of chemical devices, each such solution being peculiar to the specific type of battery involved. Venting, which has been used with many different types of cells, including zinc-halogen cells, requires periodic maintenance to make up lost material. Venting H.sub.2, e.g. in H.sub.2 and O.sub.2, may pose a safety hazard, depending on the particular situation, e.g., confined space. Not only would the cell be damaged if rupture occurred, but personnel and/or the surrounding environment could be harmed as well. For this reason, alternatives to venting have been sought and developed in the art.
For example, U.S. Pat. No. 3,305,404 (issued Feb. 21, 1967) describes the use of gas permeable membranes in lead-acid batteries, in conjunction with expensive platinum or palladium catalysts, to effect flameless combustion of hydrogen gas which would otherwise build up in the system. This approach is recommended where significant amounts of oxygen and lesser amounts of hydrogen are evolved in lead-acid systems. U.S. Pat. Nos. 3,410,725 (issued Nov. 12, 1968); 3,424,617 (issued Jan. 28, 1969); 3,598,653 (issued Aug. 10, 1971) and 3,817,717 (issued June 18, 1974) describe methods of removing hydrogen gas, e.g., by forming H.sub.2 O, from alkaline or lead acid cells by using electrodes having expensive catalysts such as platinum, palladium, silver or silver compounds. U.S. Pat. No. 3,117,033 (issued Jan. 7, 1964) describes using a hydrogen absorbing, silver compound, e.g., silver oxide, electrode for removal of hydrogen from alkaline storage batteries. U.S. Pat. No. 3,980,501 (issued Sept. 14, 1976) describes the use of hydrogen absorbing lanthanide cobalt or lanthanide nickel compound electrodes to remove gaseous hydrogen in alkaline batteries.
Of all the various techniques proposed in the prior art described, there is no recognition or suggestion of the technique used in the present invention to convert hydrogen gas in zinc-halogen compound cells and to permit a totally sealed configuration for such cells without the use of expensive catalytic devices.